FIG. 1 is a sectional view schematically showing an example of a configuration of the conventional CMUT (Capacitive Micromachined Ultrasonic Transducer)-type ultrasonic transducer.
The conventional CMUT-type ultrasonic transducer comprises a substrate 104, a vibrating membrane 105 which sends and receives an ultrasonic wave, and a vibrating-membrane supporting section 101 which is provided in one side of the substrate 104 and supports the vibrating membrane 105 so that the vibrating membrane 105 faces the substrate 104. Also, a membrane-side electrode 102 formed in the vibrating membrane 105 is disposed opposite to a substrate-side electrode 103 formed in the substrate 104.
In the CMUT-type ultrasonic transducer provided with the above-described configuration, when the vibrating membrane 105 receives an ultrasonic wave (sound pressure), the vibrating membrane 105 and the membrane-side electrode 102 vibrate, and the ultrasonic transducer sends an electric signal concerning the received ultrasonic wave, based on the capacitance change which occurs at the time of vibration between the membrane-side electrode 102 and the substrate-side electrode 103. Also, DC voltage and AC voltage are applied between the membrane-side electrode 102 and the substrate-side electrode 103, thereby the vibrating membrane 105 vibrates and the vibrating membrane 105 sends an ultrasonic wave. Such a CMUT-type ultrasonic transducer has an excellent frequency response such as a wide band, a high sensitivity.
For example, the non-patent literature 1 discloses such a conventional CMUT-type ultrasonic transducer and a manufacturing method thereof. In the CMUT-type ultrasonic transducer described in the non-patent literature 1, at the time of a later-described wet etching, a nitride layer for protecting a substrate is formed on a silicon substrate, and a so-called sacrifice layer containing polycristal silicon is vapor-deposited on the nitride layer. Then, both a vibrating membrane containing nitride and a vibrating-membrane supporting section are vapor-deposited on the sacrifice layer, and a hole for removing the sacrifice layer is formed in the vibrating membrane, and then the sacrifice layer is removed by wet etching. Subsequently, the hole is filled, a membrane-side electrode is vapor-deposited on the vibrating membrane, and then a protective layer is formed on the membrane-side electrode.